Adding unique task affinity for FallbackHome activity

Settings activities were launched and resided on home task
which was originally started by FallbackHome. Adding a
unique task affinity for FallbackHome activity in order to
prevent the task being reused by other Settings activities.

Bug: 135696366
Test: starting Settings activity before FallbackHome finishes
Change-Id: I3fe41dd3b77e37236b11006dbf08727783b6a2ec